1. Field of the Invention
This invention relates generally to methods for granulating ammonium phosphate containing fertilizer compositions, and more particularly to the combination of a kneading mill and a granulator for achieving superior granulation to that heretofore possible by conventional techniques.
2. Description of the Prior Art
Granular mixed fertilizer is the term normally used to denote fertilizers which contain some or all of the ingredients: nitrogen, phosphorus and potash. The fertilizer raw materials and ingredients are normally turned into some form of granular product by a number of different processes which are collectively called granulation processes. The various granulation processes may differ in many aspects, but the general functions are the same.
In all commercial plants using the conventional processes, numerous major operating problems occur to shut the plant down or to run up maintenance costs and operating costs, and often result in inferior product quality.
In one type of conventional fertilizer process which uses a rotary drum as the granulator, ammonia is reacted with phosphoric acid and optionally with sulfuric acid, to produce a fluid mixture or a slurry. This process is often referred to as an ammonium phosphate based process. In this process, acid feeds (e.g., phosphoric acid, sulfuric acid, etc) are partially neutralized with the ammonia in a reactor, forming an aqueous slurry. The slurry is then combined with recycle material (consisting of dry dust or finely divided particles from the tail end of the plant) and dry raw material in the granulator in such a way as to cause agglomeration of the dust and liquid into round balls. This agglomeration step could be done in a rotary granulator drum, a pug mill, or some other device which will contact the liquid and the dry materials to cause them to ball up. Material is discharged from the granulator in the form of balls, lumps and fine and dusty undersized particles. Oversized particles are crushed by a grinder and recycled with the undersize back to the granulator as dust and fragments of granules.
In conventional granulation processes, dust and crushed oversized material and some product-sized material are recycled at a ratio of from six to ten parts of recycle to every part of raw material fed to the system. This means that for every ton of product which is produced, about 7 to 11 tons of total ingredients must be put through the granulator, dryer and screening equipment. The plant capacity and equipment sizes are governed mainly by the total amount of recycle which must be returned to the granulation steps. In conventional plants this requires very high investments in equipment for the sole purpose of recycling material back through the plant, granulating the material, redrying the material, rescreening and regrinding all of the oversize material. Thus, if a new process could be developed to reduce the amount of material recycled, a major reduction in equipment investments could be realized for new plants, and existing plants could realize an increase in capacity.
It was the aim of the present invention, therefore, to develop a process which would reduce the amount of recycle and at the same time produce granules which are harder and more uniform in size and shape than could heretofore be obtained. To do this, it was the aim of this invention to consistently produce a very high percentage of "on-size" product material at the exit of the granulation step. In conventional processes, only a small percentage of the material leaving the granulation step is product size and the remainder consists of large lumps and much dust. It was felt that if a high percentage of on-size granules could be consistently produced with few lumps and less dust, an increase in plant capacity and a decrease in production costs could be realized as well as obtaining a greatly improved product quality.
Essentially, two types of equipment are used today in the granulation procedure, a pug mill and a drum granulator. Neither of these units, however, have proven to provide satisfactory results, at least neither have been successfully used to achieve the aforementioned aims.
The type of granules produced in the drum granulator leaves much to be desired. They are normally rough and irregular in shape and are also quite porous. Due to this configuration, a large amount of surface area exists per given volume of granules. This can lead to serious caking problems when the fertilizer is stored, unless the material is dried to a very low moisture content. However, when dried to a sufficiently low moisture content, other problems arise; the rough, dry granules have low physical strength. During handling by transferring equipment, rough edges on the granules break down, causing the formation of dust and fines, which must be recycled and reprocessed. The presence of dust and fines contribute to three major problems: (A) promotion of caking in storage, (B) the necessity of using sophisticated pollution control equipment, and (C) increased handling steps. During handling at storage facilities distantly located from the production plants, the caked material or fines must be returned to the production plant for reprocessing. This greatly increases the cost of the finished product.
The present inventors have observed that the reason for the wide particle size distribution of granules produced in a drum granulator is that the function of the granulator is to cause agglomeration of small particles to form larger particles. The small particles are wetted and are brought into mutual contact. The intent is that the smaller particles will agglomerate in preference to the larger particles. In actuality, however, the wetted small particles are just as likely to combine in the granulator to form larger particles, as are two wetted larger particles likely to combine to form a still larger particle. When two larger particles combine however, the result is a lump of oversized material which will eventually have to be crushed and recycled. Moreover, in general, the degree of granulation is almost totally dependent upon the amount and composition of the liquid phase present in the granulator. If the liquid phase present at a particular point in the granulation is high, overgranulation will be the more likely result. If insufficient liquid phase is present, the smaller particles will not be able to stick together.
Product normally produced from a pug mill is quite similar to that produced in the drum granulator. Pug mill produced granules are also rough and irregular, causing the same problems discussed earlier. A major disadvantage of the use of a pug mill is that a higher recycle ratio (the ratio of recycle solid material to product produced) is required as compared with the use of a drum granulator. For a given equipment size the overall plant capacity is effectively reduced as the recycle is increased. When producing the usual phosphate fertilizer grades, the drum granulator normally operates at a recycle ratio of about 5:1 to 7:1. A pug mill, however, normally operates around 10:1 recycle ratio. In addition to reducing the plant capacity, a large amount of recycle increases the dryer loading. As a result, product moisture content is higher and the risk of caking in storage is greatly increased.